Method of making a plurality of flow sensors

ABSTRACT

A method of making a plurality of flow sensors is provided, each flow sensor having a substrate with a sensing element and flow channel aligned over the sensing element. The sensing element senses at least one property of a fluid. The flow channel is aligned by one or more guide elements formed in an alignment layer. The flow channel across the sensing area is accurately and precisely aligned due to the guide elements provided at the wafer-level, facilitating reliable, low-cost, and consistent results among multiple flow sensors. The flow sensor is adapted for use in harsh environments.

This is a divisional of U.S. patent application Ser. No. 10/930,546,filed Aug. 31, 2004, entitled “Flow Sensor With Self-Aligned FlowChannel,” which is a continuation-in-part of U.S. patent applicationSer. No. 10/128,775, filed Apr. 22, 2002, entitled “Sensor Package forHarsh Environments”, now U.S. Pat. No. 6,911,894, which is acontinuation-in-part of U.S. patent application Ser. No. 09/656,694,filed Sep. 7, 2000, entitled “Robust Fluid Flow and Property MicrosensorMade of Optimal Material,” now U.S. Pat. No. 7,109,842, which is acontinuation-in-part of U.S. patent application Ser. No. 09/207,165,filed Dec. 7, 1998, entitled “Rugged Fluid Flow and PropertyMicrosensor,” now U.S. Pat. No. 6,184,773, and U.S. patent applicationSer. No. 09/368,621, filed Aug. 5, 1999, which is a Continuation-in-Partof U.S. patent application Ser. No. 09/239,125, filed Jan. 28, 1999,both entitled “Microsensor Housing,” now U.S. Pat. Nos. 6,322,247 and6,361,206 respectively. The content of the foregoing patent applicationsand patents are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to sensors utilized to detectthe quality and movement of fluids, in either gaseous or liquid form.The present invention relates more particularly to thermal sensors ofsuch fluids, such as fluid flow or property sensors implemented onsilicon, glass, quartz, or other substrates in microstructure form. Thepresent invention relates to sensor packages for harsh environments. Theinvention relates to the alignment of a flow path over a sensing area.

BACKGROUND

Flow sensors are utilized in a variety of fluid-sensing applications fordetecting the movement of fluids, which may be in gaseous of liquidform. One type of flow measurement, for example, is based on thermalsensors, which can be utilized to detect the properties of a fluid.Thermal sensors may be implemented, for example, over a siliconsubstrate in microstructure form. For convenience sake, and withoutlimitation, the term “flow sensor” can be utilized to refer to suchthermal sensors. (See e.g. U.S. Pat. No. 6,322,247 FIGS. 10a-f, and U.S.Pat. No. 6,184,773, which are both incorporated herein by reference.).The reader will appreciate that such sensors may also be utilized tomeasure intrinsic fluid properties such as thermal conductivity,specific heat (e.g. U.S. Pat. Nos. 5,237,523 and 5,311,447, which areboth incorporated herein by reference.), non-intrinsic properties suchas temperature, flow velocity, flow rate, and pressure, and otherproperties; and that the flows may be generated through forced ornatural convection.

A thermal-type flow sensor can be formed from a substrate that includesa heating element and one or more heat-receiving, or sensing, elements.If two such sensing elements are utilized, they can be positioned at theupstream and downstream sides of the heating element relative to thedirection of the fluid (liquid or gas) flow to be measured. When fluidflows along the substrate, it is heated by the heating element at theupstream side and the heat is then transferred non-symmetrically to theheat-receiving elements on either side of the heating element. Since thelevel of non-symmetry depends on the rate of fluid flow, and thatnon-symmetry can be sensed electronically, such a flow sensor can beused to determine the rate and the cumulative amount of the fluid flow.

Such flow sensors generally face potential degradation problems whenexposed to harsh (e.g., contaminated, dirty, condensing, etc.) fluids,including gases or liquids that can “stress” the sensor via corrosion,radioactive or bacterial contamination, overheating, deposits orfreeze-ups. The sensitive measurement of the flow, or pressure(differential or absolute) of “harsh” gases or liquids that can stress,corrode, freeze-up, or overheat the sensing elements is a challenge thatis either unmet or met at great expense. Among the solutions proposedpreviously are passivation with the associated desensitization of thesensor, heaters to raise the temperature of gaseous fluids to bemeasured to avoid condensation or freeze-ups (or coolers to preventoverheating) at the expense of sensor signal degradation, cost increaseand possible fluid degradation, or filters to remove objectionableparticulate matter. Frequent cleaning or replacement and recalibrationof the sensors are additional, but costly, solutions. Sensitive,membrane-based differential pressure sensors can be protected againstcontamination because no flow is involved, but they are less sensitive,typically cover a smaller flow range and are more expensive than thermalmicrosensors, in addition to not being overpressure proof.

The measurement of liquid flow via thermal microsensors, especially ofelectrically conductive fluids, thus presents challenging problems interms of electrical insulation, flow noise, chip corrosion, potentialfor leaks or structural integrity of the flow channel, and thermalmeasurement. The electrical contacts to the sensor chip generally shouldbe insulated from each other so the resistance to electrical leakage isabove approximately 20 MΩ to avoid interference with the sensingfunction. Some Si₃N₄ passivation films, for example, have pinholes;spin-on coatings of compounds that form glass or Teflon® films uponcuring have not shown insulation beyond a few days of contact with saltwater. (Note that Teflon® is a registered trademark of the E.I. Du PontDe Nemours & Company Corporation of 101 West 101 West 10^(th)St.,Wilmington, Del. 19898.) Even potting the wire-bonds in highlycross-linked epoxy led to either resistances dropping to, for example,30MΩ and/or bond breakage if the epoxy became too brittle due toexcessive cross-linking and/or thermal cycling. Additionally, an oddshape of the flow channel above the chip causes extra turbulence andcorresponding signal noise. Another approach to providing electricalinsulation for the electrical contacts and leadout wires is to move themout of the fluid-flow channel and contact area; however, such sidewisedisplacement adds real estate to the chip size and therefore to itscost.

Regarding structural integrity, a sensitive 1 μm-thick flow sensingmembrane can easily break as a result of the stronger viscous andinertial forces that a liquid can exert on it. Such breakage has evenbeen observed in cases of sharp gaseous pressure or flow pulses.Finally, with respect to thermal measurement issues, the heatertemperature rise typically permissible in liquids (e.g., ≦20° C.) ismuch smaller than the one typically utilized in gases (e.g., 100-160°C.). The resulting, relatively small signal causes more significantincreases in the effect of composition-, sensor-material- andtemperature-dependent offsets, which can cause significant errors in thesensor flow readouts.

Based on the foregoing, the present inventors have concluded that asolution to the aforementioned problems lies appropriately in the“smart” application onto the sensing chip of a film that is strongenough to function as a protective barrier to the transfer of electricalcharges and of molecular mass transfer but can be thin enough to enabletransfer of heat to allow thermal measurements. The films may befashioned of materials composed of inorganic compounds (even metals) orof hydrophobic or hydrophilic polymeric materials, as explained infurther detail herein, which can result in operational flow sensors ofhigh reliability, no electrical leakage, no fluid leakage by virtue ofthe non-intrusive character of the flow measurement, no corrosion, nofluid contamination, reduced flow noise and significantly reduced offsetand drift problems.

Another challenge in the design and manufacture of flow sensors is thealignment of the fluid flow path across the sensing element. Precise andaccurate alignment is necessary to achieve optimal performance of thesensor. Such precise alignment of sensors generally requires componentsof each sensor to be individually aligned, which is labor intensive andexpensive. Time and cost in manufacturing flow sensors is greatlyreduced when more of the production steps are completed while thesensors are at the wafer level. The present invention provides asolution to aligning the flow path precisely when the microsensors areat the wafer level.

SUMMARY OF THE INVENTION

The present invention provides a thermal sensor utilized in thedetection of the quality or properties of fluids, including gas andliquid. The thermal sensor can be implemented on silicon, glass, quartz,or other substrates in microstructure form.

In one embodiment, the flow sensor has a substrate with a sensingelement, one or more guide elements, and a flow channel; wherein theguide elements align the flow channel over the sensing element. Thesensing element senses at least one property of a fluid. In a furtherembodiment, first and second guide elements define the flow channel. Thepresent flow sensor provides a sensor in which the flow path across thesensing area is accurately and precisely aligned, facilitating reliableand consistent results among multiple flow sensors.

In another embodiment of the invention, a molded element defining one ormore flow channel extensions is positioned over the guide elements, withthe flow channel extensions in fluid communication with the flowchannel. The combination of the flow channel and flow channel extensionsdefine a fluid flow path over the sensing element. The molded elementcan form the top of the fluid flow path, or a cap can be attached to themolded element to form the top of the fluid flow path. In anotherembodiment of the invention, the flow sensor includes a substrate with asensing element, an alignment layer deposited on the substrate anddefining a location channel aligned over the sensing element, and a flowtube positioned within the location channel.

A method is provided for making a plurality of flow sensors each havinga flow channel aligned with a sensing element. The method involvesproviding a substrate with a plurality of sensing elements aligned in apattern, depositing a polymer layer onto the substrate and forming aplurality of guide elements in the polymer layer, with the guideelements positioned to align flow channels over a sensing element. Insome embodiments, the guide elements form the flow channels. Thesubstrate is then cut or diced into a plurality of pieces or chips, witheach piece having a flow channel precisely aligned over a sensingelement.

In yet another embodiment of the present invention, an apparatus isdisclosed herein for detecting liquid flow in what may generically bereferred to as a “harsh environment”, in which toxic or corrosive fluidsare analyzed. This embodiment can also be used for sensing pure orsuper-clean fluids, such that their contact with the sensor does notresult in any detectable contamination of the fluid or adverse effectsto the sensor. This improvement results from the sensor being isolatedfrom the fluid flow path.

A sensor can be configured to generally include a flow channel blockhaving a flow channel formed therein. The sensor additionally includes asubstrate fastened to a sensor chip and contacted by at least onebonding element and a molded core tube inserted into the flow channel ofthe flow channel block, which thereby reduces flow noise and potentialcorrosion, improves electrical insulation, structural integrity andthermal measurements thereof derived from the sensor chip.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, like reference numerals refer to identical orfunctionally similar elements throughout the separate views.

FIG. 1 illustrates a prior art cross-sectional view of a flow channelblock;

FIG. 2 depicts a cross sectional view of an isolated flow channel blockwith an inserted core tube;

FIG. 3 illustrates a side sectional view of an improved flow channelblock with an inserted core;

FIG. 4 depicts a graph illustrating the performance of thermal flowsensors with salt water at ambient temperature relative to a flow sensorwithout a core tube;

FIG. 5 illustrates a front view of a flow sensor in accordance with anembodiment of the present invention;

FIG. 6 depicts a cross-sectional perspective view of a temperature fieldgenerated by a flow sensor heater, in accordance with an embodiment ofthe present invention;

FIG. 7 illustrates a cross-sectional end view of a flow sensor assemblywith a glass chip under a Teflon® tube in an epoxy matrix, in accordancewith an embodiment of the present invention;

FIG. 8 depicts a graphical diagram illustrating a sensor package forharsh environments applied to large flow channels, or to propertymeasurements, which may be implemented in accordance with an embodimentof the present invention;

FIG. 9 illustrates a sectional top view and a bottom view of a flowsensor assembly with a small core tube located within walls of a flowchannel block thereof, in accordance with an embodiment of the presentinvention;

FIG. 10 depicts sectional views of an assembly of a flow channel blockand a core tube, in accordance with an embodiment of the presentinvention;

FIGS. 11A and 11B are front and top views, respectively, of amicrosensor assembly with an alignment layer according to the invention;

FIG. 12 is a perspective view of another embodiment of microsensoraccording to the invention;

FIGS. 13A and 13B are front and top views, respectively, of a furtherembodiment of microsensor according to the invention;

FIGS. 14-17 are front cross-section views of other embodiments of theinvention in which guide elements provide alignment for a flow channelor flow tube;

FIG. 18 is a graph illustrating the performance of thermal flow sensorsmade of different materials and having different wall thicknesses (WT),with salt water at ambient temperature relative to a flow sensor; and

FIG. 19 is a graph illustrating the performance of a stainless steelflow tube with and without oil added to the junction between the flowtube and microbrick.

DETAILED DESCRIPTION

The particular values and configurations discussed in these non-limitingexamples can be varied and are cited merely to illustrate variousembodiments of the present invention and are not intended to limit thescope of the invention.

One aspect of the present invention is related to the design andfabrication of the electrical insulation for electrical contacts tosensor chips using either front-wire-bond (FWB) or through-the-wafer(TTW) contacts of certain thermal flow microsensors or of environmentalsensors in general. The present inventors previously insulated Au-wiresand Au-pads of FWB sensor chips via materials, such as, for example,dip-coatings, dip-coatings with or without alumina thin-filmundercoating, Si₃N₄, flowable sealants, solvent-resistant sealant withfluoro-silicon, and epoxies. Insulation based on such materials has beenattempted as defined generally by the resistances between the sensingelements and the liquid (e.g., salt water) in a flow tube. Suchresistances, however, are unacceptable if ≦20 MΩ. The inventiondescribed herein thus introduces a unique solution for solving suchproblems.

As will be explained in further detail herein, by potting insulatingmaterial (e.g., epoxy) around a core-mold of Teflon® wire or pipe of0.010 to 0.060″ OD, which may or may not be removed after curing, andusing for example, a robust microbrick or an epoxy-back-filledmicrobridge, the aforementioned problems can be essentially eliminated.The increased thickness of the insulating “layer”, relative to adip-coat for example, causes the intrusion of fluids (e.g., water) andother conductive materials, such that their contribution to electricalconduction in the polymer becomes negligible. A straight and smooth flowchannel, which can reduce turbulence and flow noise, thus replaces theold flow channel spaces located above previously utilized sensor chips.

Replacing an unprotected microbridge by a microbrick chip can eliminatebreakage due to fluid-generated forces. Note that the utilization of amicrobrick chip or other such devices are not considered limitingfeatures of the present invention but are mentioned herein forillustrative and general edification purposes only. The increasedinsulation thickness enables the application of larger voltages to thesensor heating elements, which raises the heater temperature (which mayor may not be in direct contact with the liquid) and leads to largeroutput signals. As a result, heater resistance drift, and temperature-,fluid-type-, sensor-asymmetry-, and electronics-dependent offsets areless prominent.

In one embodiment, a flow sensor includes a flow channel block defininga flow channel, a molded core tube positioned within the flow channel, asubstrate, a bonding element, and a sensor element or chip. The bondingelement can be configured to comprise one or more front wire bonds(FWBs) and/or through-the-wafer (TTW) contacts.

As used herein, the term “tube” means a conduit or channel of any shapethrough which a fluid flows. The cross section of the tube can becylindrical, polygonal, elliptical, etc. The molded core tube can beformed from a polymeric material, such as Teflon®, or other materials,such as glass, quartz, sapphire and/or metal, such as, for example,stainless steel. The tube can be made of a mixture of different plasticsor polymers. The molded core tube generally comprises a wall thicknessthat removes a surface of the sensor chip from direct contact with afluid flowing through the molded core tube by a distance correspondingto the wall thickness, thereby desensitizing the sensor to fluid flowvariations. Additionally, this tube wall thickness in contact with thesensor chip combines a high dielectric strength and chemical inertnesswith properties such as hydrophobic, hydrophilic and lipophilic asneeded. Such properties may be realized with inorganic or organicmaterials. Note that as utilized herein the term fluid can be meantgenerally to refer to a gas or liquid. Thus, sensor packages disclosedherein can be utilized to measure the quality or property of a gas or aliquid.

The film can be enlarged to comprise a potting or molding compoundassociated with the bonding elements, whereby the molded core tubegenerally shapes the potting compound. The film itself may be formedfrom a material such as, for example, an epoxy material. Also, themolded core flow channel can be configured to include a constriction ina cross section of the molded core tube at the sensor chip to optimizeperformance thereof. The molded core flow channel and the substrate canbe replaced by a flat film, which can be wrapped or shrunk about aheader and sealed by an O-ring to provide sensor capabilities thereof.The flow tube is generally configured from a flow channel block and canbe a disposable flow tube. Additionally, the sensor can be associatedand/or integrated with a heat sink mechanism for heat sinking areference resistance and/or temperature sensor associated with aboveflow sensor so that the flow sensor does not increase in temperature anddrive an associated heater temperature to a point where a fluid flowingthrough the flow channel boils.

The sensor features a flat, passivated, top surface overlying the heaterand sensor elements to provide appropriate electrical isolation.Further, the die, with its through-the-wafer interconnections,eliminates the need for bonding wires with their attendant problems asdiscussed above. In order to withstand a wide range of pressure levelsand operate in harsh environments, the die structure is configured to bevery robust. The die is made up of materials that have very low thermalconductivity, thus eliminating the possibility of undesired thermalsignal shorts. For example, the die can be fabricated using variousglass materials, alumina, or combinations of such materials.

The ability to perform high mass flux sensing operations is largelydependent upon the physical characteristics of the sensor. Mostimportantly, low thermal conductivity of the die substrate is necessaryin order to create a sensor capable of operating in these high mass fluxsensing situations. By minimizing the thermal conductivity, interferencewith sensor heating/cooling effects will be minimized and the sensingcapabilities are enhanced. Specifically, the characteristics of the diesubstrate materials will control the proper route of heat transfer,avoiding transfer through the die substrate from the heater to thesensors. Various materials can provide this characteristic.Historically, silicon nitride of a microbridge sensor chip has been usedto provide certain levels of thermal conductivity, while also beingeasily manufactured. However, its fragility prevents is use in harshenvironments.

A more optimum material that exhibits the desired characteristic isglass. Glass, however, has not been previously used because it has notbeen easily micromachined. That is, it is difficult to form the requiredstructures using glass. Another potential substrate material is alumina,which is widely used for electronics packaging and can be machined toserve as substrate with some desirable characteristics. One undesirablefeature, however, is its high thermal conductivity, which would severelyreduce the sensitivity of the sensor chip.

Recent developments in glass materials, including photosensitive glassand Pyrex®, have shown that micromachining is possible and extremelyeffective. Consequently, this material can now provide an alternate diesubstrate for a micromachined flow and property sensor. The presentinvention exploits the characteristics of glass (photosensitive glass,fused silica, etc.) or alumina materials to produce a flow and propertysensor with optimized physical characteristics. Providing a glass basedsensor in a microbrick structure or microfill structure consequentlyenables the fabrication of a rugged sensor for sensing liquid propertiesor high mass flux fluid flow, without pressure-stress-induced errorsignals.

Due to the recent developments in glass, the use of this material as adie substrate generally reduces the amount of structural machiningnecessary. More specifically, the substrate can now be fabricated in amicrobrick structure or microfill structure, which has a substantiallysolid structure. In this type of sensor die, the heating and sensingelements are placed directly on the substrate and no further processingor structuring is required beneath those elements. Consequently, thesubstrate itself is continuous beneath the sensing elements creating amore robust sensor die. The characteristics of the glass substratematerial allow this microbrick structure to be effectively used in harshenvironments.

Flow sensors are either non-isolated, in which the fluid flows directlyover the sensing element, or isolated, in which the fluid flow isseparated from the sensing element. FIG. 1 illustrates a prior artcross-sectional view 100 of a plastic non-isolated flow channel block104. FIG. 1 further illustrates a sensor chip 106 which is fastened orin communication with a substrate 102. The substrate 102 can supportelectrical I/O lead-outs, which may in turn be connected or bonded tovarious elements on chip 106 via “front wire bonds” (FWBs) 107 or“through-the-wafer” (TTW) contacts (not shown). A top flow channel 111with an appropriate opening for the chip can then be fastened over thesensing chip 106. Ideally, care should be exercised so as not to spillexcess adhesive into the path intended for the fluid. Thus, view 100represents a drawing of a microsensor, prior to the introduction of the“core mold” concept of the present invention, as explained in furtherdetail herein.

FIG. 2 depicts a cross sectional view 110 of an isolated flow channelblock 104 with an inserted core tube 118. FIG. 2 additionallyillustrates a sensor chip 116 and a substrate 112. Flow channel block114, which is analogous to flow channel block 104 of FIG. 1, nowpossesses an inserted core tube 118. Substrate 112 may be composed of,for example, alumina, PCB, glass, quartz, or other substrate-typematerials. Substrate 112 of FIG. 2 is generally analogous to substrate102 of FIG. 1. Note that the term “substrate” as utilized herein canrefer to a “substrate” or a “substrate board.” The composition of thesubstrate is discussed further below. The flow channel block 114 is alsogenerally analogous to flow channel block 104, with the exception thatcore tube 118 has been added to block 114. This facilitates the processof fastening flow channel block 114 to an “alumina” substrate 112.

The inserted core tube 118 is not pulled out but is maintained in placeto provide the above-discussed advantages. Note that the wall thicknessof inserted core tube 118 removes the surface of sensing chip 116 fromdirect contact with the fluid by a distance corresponding to thatthickness, thus desensitizing the sensor to flow changes, which is theprice paid for the other benefits mentioned above. Additionally, it isimportant to note that flow channel block 114 may be configured in theshape of a tube, thereby functioning as a flow tube. Flow channel block114 thus may form a flow tube.

It can be appreciated by those skilled in the art, however, that flowchannel block 114 may be configured in the form of other shapes, such asfor example, a triangular-, square-, rectangular-shaped flow channelblock, half-circles, or various other geometric shapes. Thus, the shapeof flow channel block 114 can be an arbitrary design choice and is notconsidered a limiting feature of the present invention. Additionally, itcan be appreciated that flow channel block 114 can be formed from avariety of materials, including, but not necessarily only, plastic.

In one embodiment, flow channel block 114 is a polymer alignment layerdefining a location channel into which core tube 118 is inserted. Thealignment layer provides a location channel precisely aligned oversensor chip 116 and allows core tube 118 to be precisely aligned overthe sensor chip 116. The composition of an alignment layer is discussedfurther below.

Substrate 112 can support electrical I/O lead-outs, which may in turn beconnected to various elements on sensor chip 116 via “front wire bonds”(FWBs) 127 and 129 illustrated in FIG. 2. Similarly, FWBs 107 and 109are depicted in FIG. 1. Additionally, bonding elements can be configuredas through-the-wafer (TTW) contacts, which are not illustrated in FIGS.1 and 2. Flow channel block 114 can then be fastened over sensing chip116 and substrate 112. Ideally, care should be exercised so as not tospill excess adhesive into the path intended for the fluid in FIG. 1. InFIG. 2, core tube 118 can prevent such spills and generally surroundschannel 121 through which a fluid may flow. Note that if core tube 118is removed from flow channel block 114, channel 121 can be left in placeafter core tube 118 is removed from molding surrounding core tube 118.In this sense, core tube 118 may also be referred to as a “molded coretube.”

The use of such a core tube can thus reduce flow noise, sensitivity, andthe risk of contamination of super-clean fluids, fluid leakage, chipcorrosion and leakage potential, while improving electrical insulation,structural integrity and thermal measurements thereof derived from anassociated sensor chip (e.g., sensor chip 116). Such a core tube canalso be used to shape and mold an inner flow channel, which can beremoved after curing of the molding compound. The flow sensor can thenregain flow sensitivity and maintain low “flow noise” but may lose somechip corrosion protection, fluid and electrical leakage prevention,fluid contamination, non-intrusiveness and structural integrity.

Again, referencing FIG. 2, the substrate 112 can be comprised ofalumina, mullite, quartz, or other known materials having coefficient ofthermal expansion (CTE) suitably matched to the microsensor die. Siliconis often considered a very effective microsensor body material becauseit can be easily machined/processed using several well-known siliconprocessing techniques. In certain applications, such as very high massflux fluid flow sensing and high-pressure applications, such siliconsupported structures as microridges or micromembranes do have certaindisadvantages however. Specifically, the thermal isolationcharacteristics of silicon would limit structural and operationalcharacteristics of a sensor if built directly on silicon. In order todeal with these thermal characteristics, the microsensor body of asilicon-based sensor is configured in a micromembrane type structure, soas to limit the thermal mass below the heater and sensing elements.Obviously, this limits the physical strength of a silicon-based sensor.In addition, this micromembrane configuration is not suitable for highmass flux sensing because its output signal saturates before reactinghigh flux levels.

In order to effectively operate in harsh environments, the flow sensormust be structurally robust. What is needed is a sensor robust enough towithstand high pressures due to sources (such as high pressure pulses,ultrasonic cleaning, and water hammer). In order to sense high mass fluxflow rates, it is also important to have a substrate material with anappropriately low thermal conductivity (≦1.5 W/(mK)). Certain glassmaterials provide better thermal isolation characteristics (thansilicon), thus increasing the sensing capabilities of the above-outlinedmicromachined flow and property sensor. The use of glass also allows fora more robust physical structure to be used. These variouscharacteristics result in a more versatile sensor, which can be used inmultiple applications. Furthermore, as outlined below, certaintechniques provide for effective micromachining of glass basedsubstrates.

The use of glass as a microsensor body material provides multiplefeatures that enhance the capabilities of the sensor. These featuresinclude: (1) the automatic electrical insulation for through-the-wafercontacts, (2) lower thermal conductivity than silicon, (3) environmentalruggedness needed to withstand pressure pulses as for sensing liquids,and (4) the ability to use a structurally robust sensor bodyconfiguration. Furthermore, the glass-based sensor meets allrequirements for chemical inertness, corrosion resistance, andbiocompatibility.

As mentioned above, glass provides inherent electrical isolation betweenvarious contacts. This is compared with a silicon based sensor whereelectrical isolation is achieved by incorporating silicon dioxide layerson the substrate unless more costly silicon wafers are used that aregrown to be slightly insulating. Obviously, this eliminates one layer ofmaterial and one necessary processing step. This is particularlybeneficial as the step of growing oxide is time consuming and done atfairly high temperatures.

While the sensor of the present invention can be implemented asglass-based sensor, it is understood that other materials havingappropriate physical characteristics could also be used. For example,the substrate can be manufactured out of other materials includingglass, quartz, silicon, alumina, or ceramic.

FIG. 3 illustrates a side cross-sectional view 131 of an improved flowchannel block with an inserted core, in accordance with an embodiment ofthe present invention. The left side of view 131 further illustrates aside-sectional view of the prior art configuration illustrated in FIG.1, while the right side illustrates the position of the core tube 118.Note that in FIGS. 1 to 3 analogous or like parts are generallyindicated by identical reference numerals. For example, flow channelblock 104 of FIG. 1 is analogous to flow channel block 114 of FIG. 2.Thus, as indicated in view 131 of FIG. 3, walls 122 and 124 form wallsof flow channel blocks 104 and 114.

FIG. 3 is included herein primarily to highlight the differences betweenthe prior art configuration depicted in FIG. 1 and the improved flowchannel block design illustrated in FIG. 2. A dashed line 140 in FIG. 3indicates a separation point between the prior art design of FIG. 1 andthe improved design of FIG. 2. Thus, half of sensor chips 106 and 116are illustrated in FIG. 3, along with half of substrates 102 and 112. Achip top view 142 is also indicated, showing respective halves of sensorchips 106 and 116. As indicated above, walls 122 and 124 form walls offlow channel blocks 104 and 114. Both flow channel blocks 104 and 114include walls 122 and 124. Walls 122 and 124 are indicated on both sidesof dashed line 140. An arrow 144 indicates a flow of fluid throughchannel 111 and 121. Walls 130 and 132 of inserted core tube 118 of FIG.2 are also depicted in FIG. 3.

As explained previously, the wall thickness of the tube removes thesensing chip surface from direct contact with the fluid by a distancecorresponding to that thickness, thus desensitizing the sensor to flowchanges. This effect can be minimized and possibly balanced byincreasing the temperature of the heater temperature above an ambientlevel, and additionally by designing the wall thickness at the sensorchip contact surface to be as small as possible. Note that even with theuse of TTW contacts, the suggested use of a core pipe, whether left inplace or not after bonding the “clear plastic” part with the “alumina”,reduces flow noise and the risk of leakage or corrosion and enables theapplication of higher heater temperatures, which also leads to highersensor temperatures and reduced offsets. Note that as utilized herein,the term “bonding” generally connotes electrical contacting with thewire bonds (e.g., FWB), while the term “fastening” generally connotesmechanical securing elements and techniques thereof.

In prior art devices and systems, companies such as for example, UnitInstruments, Emerson Inc. and others, have marketed mass flowcontrollers based on thermal flow sensors with macroscopic core tubes ofstainless steel for decades. Such devices typically feature the heaterand sensing elements in the form of wire windings around the core metaltube. This fabrication approach, however, can result in large,slow-responding and costly sensors and is generally an ineffectivesolution.

Flow sensors, including the overall structures depicted in FIGS. 2 and 3can be thus designed, especially as the diameter of the core tubedecreases, thereby resulting in more favorable surface-to-volume tuberatios. In the embodiment illustrated in FIG. 2, for example, anapproximately 0.061″ OD Teflon® tubing (i.e., normally used aswire-insulation) can be threaded through the “clear plastic” flowchannel 121 cross-sectionally at the sensor chip 116. Either epoxy orRTV can then be injected via a syringe hole towards the chip area untilexcess spills out, while the unsealed alumina substrate to flow channelblock interface remains under vacuum to minimize trapped air bubbles.

Another fabrication technique can also be implemented, in accordancewith an another embodiment of the present invention, in which excessadhesive is generally applied to the individual parts prior to joining,evacuating and thereafter bringing the parts together, while squeezingexcess adhesive from the bonding surfaces. After curing of the adhesive,the Teflon® core tube can be easily removed, if desired. Measurement ofthe electrical resistance between the sensing elements and theintroduced conductive aqueous salt solution indicates resistancesbetween an initial ≧200 MΩ and subsequently after several days, ≧30MΩ,with the Teflon® tube removed. No degradation or electrical leakage maybe measured if the tube can be left in place.

FIG. 4 depicts a graph 200 illustrating the performance of flow sensorswith salt water at ambient temperature, in accordance with an embodimentof the present invention. FIG. 4 is presented for illustrative andedification purposes only and is thus not considered a limiting featureof the present invention. Graph 200 indicates that measured flow sensoroutput versus flow for several flow channel configurations and heatertemperature values can be obtained. As illustrated in graph 200, flowsthat occur below 0.5 nL/s are measurable for a smaller core tube of only150 μm internal diameter. In such instances, noise levels may beapproximately in the 1 mV range, for which no compensation forfluctuations in ambient temperatures may be in place. Those skilled inthe art can thus appreciate that graph 200 illustrates a range of datacollected over time regarding nulled-output versus flow rate. Graph 200thus generally illustrates the beneficial influence of lower wallthickness (WT) and higher thermal conductivity materials for the coretube, which increases sensitivity and flow ranges. An example of ahigher thermal conductivity material, which may be utilized inassociation with an embodiment of the present invention, is Pyrex®.(Note that Pyrex® is a registered trademark of the Corning Glass WorksCorporation of Corning, N.Y. 14831.) A further explanation of FIG. 4 isthus not necessary.

FIG. 5 illustrates a front view of a flow sensor 300 that can beimplemented in accordance with another embodiment of the presentinvention. Flow sensor 300 includes an outer surface 310 and an innersurface 312 of a core tube 308 located above a sensing chip 302 withFWBs 306 and 304.

FIG. 6 also depicts a cross-sectional side view of a temperature field400 generated by a flow sensor heater, in accordance with an embodimentof the present invention, whereby the heater can be raised to anexemplary 100° C. above ambient in a plane just 25 μm off the centerwith no flow present. FIG. 6 generally illustrates the results of afinite-element computation of the temperature profile of a temperaturefield near the sensor chip (e.g., sensing chip 302 of FIG. 5 or chip 116of FIG. 2), thus indicating that even the ΔT_(h)=6.5° C. isotherm barelypenetrates the water and accounts for the loss in sensitivity if thethickness of the flow channel block can be chosen to be as large as, forexample, 250 μm. The use of thin-wall tubes, made of materials of higherthermal conductivity (e.g., approximately 1 W/(mK)) has beendemonstrated as a valid approach to minimizing the sensitivity loss.

It can be appreciated that modifications to the aforementioned improvedsensor configuration (i.e., sensor package) can be made in accordancewith the present invention. For example, heat sinking a referenceresistance, R_(r), which is not shown in FIG. 6, to achieve propercontrol of an associated heater, can be implemented by the skilled whenfamiliar with the SoA. Choosing a thin but strong core tube, made ofmaterial with intermediate thermal conductivities is another techniquethat can be utilized, as described above, in accordance with theapparatus of the present invention. Other variations and alternativeembodiments are further described below.

FIG. 7 illustrates a cross-sectional end view 500 of a flow sensorassembly 505 with a glass chip 510 under a Teflon® tube 516 in a flowchannel body or block 520 (e.g., of about 0.25×0.25″ in cross section),which may be implemented in accordance with a further embodiment of thepresent invention, and which can be sized to fit into a Honeywell flowchannel housing AWM720. As illustrated in FIG. 7, chip 510 can belocated above a substrate 508, which may be composed of alumina, glass,or other substrate material. Chip 510 can be configured to include FWBcontacts to substrate 508 via wires 522 and 524. Core tube 516 can be0.060″ in diameter. Additionally, a 0.002″ wall thickness can beutilized to sense water flows between <10 to >1000 μL/min. The smallercore tube 514 can be inserted into a groove in rod 512, in place of coretube 516 and may be utilized to sense the flows illustrated in FIG. 4 ina range of, for example, 0.03 μL/min to 3 μL/min. Note that rod 512 ispositioned generally above block 521 in FIG. 7. Block 521 is in turnpositioned above substrate 508.

FIG. 8 illustrates a sensor package for harsh environments applied tolarge flow channels, which may be implemented in accordance with anembodiment of the present invention. The sensor chip 616 can be fastenedonto a header (e.g., #T08 614 or #T05 612) and electrically bonded toone or more associated posts via wires 617 and 619. Instead of exposingthe chip surface to the fluid flowing in a channel as large as 0.5″ ormore, the sensor chip can be protected by film 610, which may becomposed of any number of single or laminated, organic or inorganicmaterials. The thin film 610 is applied to the sensing element, whereinsaid thin film is applied thinly, thereby enabling reliable, sensitive,low-noise, non-intrusive, non-contaminating, and flow-channel-disposablemeasurements thereof. In one embodiment, for example, the thin film isapplied to the sensing element at a thickness in an inclusive range fromabout 0.001-in to about 0.010-in.

Note that a virtual channel 606 is depicted in dashed lines in FIG. 8.Such a virtual channel 606 may be, for example, approximately 0.060-inin diameter. As illustrated in FIG. 8, voids 602 may be filled withadhesive such as epoxy. An underside 604 of the polymer film 610 may be“etched” to promote adhesion. An O-ring 618 can be placed around thebase of the header to enable sealing against the fluid in the largechannel. The header can be fastened by known fastening techniquesagainst the large flow channel block 608, of which only a corner isillustrated in FIG. 8.

FIG. 9 illustrates a sectional top view 901 and a bottom view 900 of aflow sensor assembly with a small core tube 918 (e.g., of ˜0.014″ outerdiameter and 0.006″ inner diameter) located within walls 906 and 908 ofa flow channel block thereof, in accordance with an embodiment of thepresent invention. Note that top and bottom views 901 and 900 areseparated from one another in FIG. 9 by a dashed line 903. Duringassembly of the structure illustrated in FIG. 9, an epoxy can beutilized to fill all voids except the inner diameter of core tube 918.Pusher element 910 can be utilized to press core tube walls 912 and 914onto sensor chip 920 to minimize any void between sensor chip 920 andtube wall 914. This design simplifies for some applications the assemblyof small core tubes as explained herein (e.g., the small core tube ofFIG. 7). Thus, a simplified yet efficient core tube structure for usewith sensor packages for harsh environments can be readily constructed,particularly in view of commercially available parts (e.g., block walls906, 908, and sensor chip 920, wherein wall 906 can also comprise thecircuit bearing substrate).

FIG. 10 depicts sectional views 1000 and 1001 of an assembly of a flowchannel block 1119 and a core tube 1120, in accordance with anembodiment of the present invention. View 1000 illustrates an epoxyadhesive 1009 located beneath a film 1008 (e.g., Teflon® tape), whichcan encase one or more FWBs. During curing and after the insertion ofepoxies 1009 and 1121 and assembly thereof, a core tube 1120 can beplaced above film 1008 and pressed via a pusher element 1002 onto asensor chip 1130 and a substrate 1006. Note that pusher element 1002 ofFIG. 10 is similar to pusher element 910 of FIG. 9.

By making certain that film 1008 does not adhere to substrate 1006 andsensor chip 1130, nor to epoxy 1121 and flow channel block 1118, one cantake the top and bottom halves apart after the epoxy has been cured andremove the film as well. Film 1008 can be formed from a material suchas, for example, a Teflon® fluoropolymer or Aclar®. (Note that Aclar® isa registered trademark of the Allied Chemical Corporation of MorrisTownship, N.J.) The structure indicated in FIG. 10 can thus befabricated, thereby permitting the perfectly mated top and bottom halvesto be reassembled, such that the surface of core tube 1120 contacts thesensing surfaces of sensor chip 1130. After completion of themeasurements, the top half of the assembly illustrated in view 1001 ofFIG. 10 can be discarded (e.g., it may contain a blood or otherbiological fluid), without having contaminated the non-disposable andgenerally more costly part holding the sensor chip 1130 and itscalibrated circuit on substrate 1006.

Based on the foregoing it can be appreciated that a number ofalternative sensor configurations can be implemented in accordance withthe present invention to achieve electrical insulation for liquid or“harsh environment” sensor chips. For example, covering a “to-be-sealed”sensor chip to sense liquid flow or liquid properties with a film thatcombines high dielectric strength and chemical inertness withhydrophobic properties, whether inorganic or not, may be utilized toachieve such electrical insulation.

Another technique for achieving electrical insulation for liquid or“harsh environment” sensor chips, in accordance with the presentinvention disclosed herein, involves enlarging and shaping the film as apotting compound of the wire-bonds around the chip, whereby thepotting-sealant-adhesive (e.g., epoxy, RTV, etc.) can be shaped by aremovable mold core (such as thin tubing or film of fluoropolymer, glassor metal) to reliably provide minimum insulation, while maximizingsensing performance (e.g., higher signal reliability/accuracy due toreduced offsets, lower-noise, longer service life, etc). In such aninstance, the tubular mold core tube may be left in place as insulationafter potting. The flow sensor itself, according to the presentinvention disclosed herein, thus can be exposed to the fluid, becausethe core tube (i.e., core flow tube) can be removed after using it tomold the flow channel. Alternatively, the flow sensor may also beexposed to the fluid if the core tube is left in place. The core tubethus may comprise a disposable flow tube.

In addition, smartly performing the potting enables the fabrication ofdisposable flow tubes (e.g., for blood or chemical analysis) withoutdisposing of the calibrated sensor and its electronics. Additionally, aconstriction in the cross section of the core tube can be provided atthe site of the sensor chip (e.g., see FIG. 1) to optimize performanceat the location of the highest flow velocity (and signal) and governingpressure drop (i.e., to minimize overall Δp).

Furthermore, the tube and the flat substrate can be replaced by a flatfilm (e.g. 20-100 μum thick Teflon®) wrapped or shrunk around a headersuch as, for example, a TO5 or TO18, and sealed by an O-ring 618 asshown in FIG. 8. Finally, as indicated previously, heat sinking thereference resistance, R_(r), so that it does not heat up andaccidentally drive the heater temperature too high and boil the liquidcan be utilized to achieve electrical insulation for liquid or “harshenvironment” sensor chips. For example, a small metallic thermalconductor may be utilized, which can be epoxied onto the R_(r) andincrease its heat exchange surface in a direction away from heaterresistance, R_(h). Another aspect of the present invention is related tothe alignment of flow channels over a sensor chip. The use of analignment layer creating a location channel provides another techniquefor achieving electrical insulation for “harsh environment” sensorchips.

While many different materials may be used to make a fluid flow sensor,the choice of material can drastically affect the sensor's performance.A suitable material for the sensor substrate would have a relatively lowthermal conductivity. A low thermal conductivity is important tomaintain the sensitivity for the sensor. With a relatively low thermalconductivity, all heating/cooling effects presented to the varioussensing elements are caused predominantly by the fluid to be sensed.Stated alternatively, it is important to ensure that heat is nottransmitted through the substrate excessively, resulting in signalshorts.

In addition to the above referenced thermal characteristics, it ishighly desirable for the overall flow sensor to be chemically inert,corrosion resistant, highly temperature stable, electrically isolated,and biocompatible. The sensor features a flat top surface overlying theheater and sensor elements to provide appropriate electrical isolation.The top surface of the sensor can be passivated. The heater and sensorelements are embedded in or attached to a substrate, or die. The sensorcan be configured to include one or more front wire bonds and/orthrough-the-wafer contacts. Through-the-wafer interconnections eliminatethe need for bonding wires. The substrate is made up of materials chosento have a relatively low thermal conductivity, thus eliminating thepossibility of undesired thermal signal shorts. For example, thesubstrate may be fabricated using various glass materials, silicon,alumina, quartz, ceramic, polymers, metal, or combinations of suchmaterials.

As shown in FIGS. 11A and 11B, the sensor 2010 includes substrate 2020,sensing element 2070, guide elements 2030 formed in an alignment layer,a flow channel 2040 defined by the guide elements 2030, and moldedelement 2050 defining flow channel extensions 2080, 2081. Substrate 2020has notches 2060 cut into the top surface to accommodate the moldedelement 2050. Flow channel 2040 is aligned over sensing element 2070,which includes a heater 2090 and thermal sensors 20100, 20101. Thesubstrate 2020 can be any conventional material used for microsensors,including silicon, ceramic, metal, glass, such as Pyrex®, or quartz.

The fluid flow path, indicated by the arrow in FIG. 11B, is preciselyaligned over the sensing element 2070 in the sensor 2010 shown in FIGS.11A and 11B through the use of guide elements 2031 defining flow channel2040. The alignment layer is a polymer material deposited over an entirewafer containing multiple sensing elements 2070 arranged in a pattern.In one embodiment, the polymer is a positive resist such as poly(methylmethacrylate) (PMMA). In another embodiment, the polymer is an epoxybased negative resist. One such resist is SU-8, which is sensitive to UVradiation, and is thermally and chemically stable after development. Thealignment layer is masked, and guide elements 2030 are formed byphotolithography.

The guide elements 2030 are positioned adjacent the sensing elements andserve to guide or align a flow channel over each sensing element. In oneembodiment, shown in FIG. 17, a single guide element 20630 is positionedadjacent the sensing element 20670, and a molded element 20650 having aflow channel 20680 is positioned over the guide element 20630 to alignthe flow channel 20680 over the sensing element 20670. In otherembodiments, the alignment layer is masked such that the guide elementscreate channels with vertical walls aligned over the sensing elements.In one embodiment, shown in FIGS. 11A and 11B, the alignment layer ismasked to create guide elements 2030 on either side of the sensingelement 2070. In this embodiment, the guide elements 2030 form a flowchannel 2040 that serves as the flow path. In other embodiments, shownin FIGS. 13-16, the guide elements form a location channel 20240, 20340,20440, 20540 into which is placed a flow tube 20285 (FIG. 13A), moldedelement 20350, 20450 (FIGS. 14, 15), or both a molded element 20550 andflow tube 20585 (FIG. 16). The flow tube or molded element in theseembodiments functions as the flow path.

In another embodiment, the substrate is a silicon wafer and the guideelements are etched using a procedure such as deep reactive ion etching(DRIE). In a further embodiment, the guide element is a V-groove formedby an anisotropic etch of KOH and water. A truncated V-groove having aflat bottom of etch resistant boron doped silicon formed initiallybeneath a layer of epitaxially grown silicon can also be used.

Forming the guide elements precisely aligned adjacent to the sensingelements while processing is still at the wafer level allows formultiple sensors to be manufactured with identically aligned flow paths.The wafer is diced into individual sensors, and molded elements or flowtubes can be positioned over the guide elements to provide a fluid flowchannel precisely aligned over the sensing element.

In the sensor shown in FIGS. 11A and 11B, guide elements 2030 define theflow channel 2040, which is also the flow path. The substrate 2020 formsthe bottom of the flow path, the guide elements 2030 form the walls ofthe flow path, and molded element 2050 forms the top of the flow path.Molded element 2050 includes flow channel extensions 2080, 2081 thatconnect to the ends of the flow channel 2040 to provide an interfacewith a fluid system. Molded element 2050 can be attached to thesubstrate 2020 with an adhesive. The guide elements 2030 provide abarrier to prevent migration of the adhesive into the flow path or ontothe sensing element. In this embodiment, multiple sensors withidentical, precisely aligned flow paths can be produced with minimalpost-wafer processing.

Another embodiment of sensor is shown in FIG. 12. Substrate 20120 has asensing element 20170. A single oval guide element 20130 is formed inthe alignment layer and defines a flow channel 20140 aligned oversensing element 20170. Molded element 20150 is configured to fit overguide element 20130 and form the top of the flow channel 20140. Moldedelement 20150 includes flow channel extensions 20180, 20181 that connectwith the ends of the flow channel 20140 at an angle and provide aninterface with a fluid system. In some embodiments, flow channelextensions 20180, 20181 have the same dimensions. In other embodiments,the two flow channel extensions 20180, 20181 are differently sized. Forexample, the flow channel extensions may have different inner diametersto accommodate the tubing or flow paths in the larger fluid flow systemin which the flow sensor is placed. In sensors having one or more anglesin the flow path, the sensing element 20170 is positioned at a distancefrom the last upstream angle that is greater than or equal to ten timesthe diameter of the flow channel. This spacing allows turbulence in thefluid caused by the angle to dissipate before the fluid passes over thesensing element.

The flow sensor embodiments shown in FIGS. 11A, 11B, and 12 arenon-isolated sensors. The fluid flow path is directly over the sensingelement 2070, 20170 and is bound by the substrate 2020, 20120 on thebottom, the guide elements 2030, 20130 on the sides, and the moldedelement 2050, 20150 on the top. The flow sensors shown in FIGS. 13A,13B, and 14-16 are isolated sensors.

The flow sensor 20210 shown in FIGS. 13A and 13B includes a substrate20220, an alignment layer defining guide elements 20230 that form alocation channel 20240 over a sensing element 20270, a molded element20250 including flow channel extensions 20280, 20281, a flow tube 20285,and a cap 20200. The guide elements 20230 are precisely aligned to formthe location channel 20240 over the sensing element 20270 and serve toalign the molded element 20250 and flow tube 20285 over the sensingelement 20270. The molded element 20250 provides additional support forthe flow tube 20285. Cap 20200 serves to hold the flow tube 20285 incontact with the sensing element 20270. The flow tube 20285 provides anisolated flow path over the sensing element 20270.

The flow tube 20285 has a wall thickness that removes a surface of thesensor from direct contact with a fluid flowing through the flow tube bya distance corresponding to the wall thickness, thereby desensitizingthe sensor to fluid flow variations and protecting the sensor from whatmay generically be referred to as a “harsh environment.” A harshenvironment may include fluids that are contaminated, dirty, condensing,corrosive, radioactive, etc. Also included are fluids that may overheat,leave deposits, or freeze up the device. The cross section of the flowtube can be cylindrical, polygonal, elliptical, etc. In someembodiments, the flow tube 20285 is disposable, providing a flow sensorthat is reusable for multiple contaminated fluid samples, such as blood.To change the flow tube 20285, the cap 20200 is removed, the used flowtube is replaced with a new flow tube and the cap is replaced.Additionally, this tube wall thickness in contact with the sensorcombines a high dielectric strength and chemical inertness withproperties such as hydrophobic, hydrophilic and lipophilic as needed.Such properties may be realized with inorganic or organic materials.

In some embodiments, cap 20200 includes a protrusion 20205 sized toextend downward to hold smaller flow tubes 20285 in contact with thesensing element 20270. The sizes of the molded element 20250 and cap20200 can be selected to provide stability for various sizes of flowtubes 20285. In this way, multiple sensors cut from a single wafer, eachwith identical sized location channels 20240, can be used with differentsizes of flow tubes 20285. Additionally, the molded element 20250 canextend into the location channel 20240 to provide a narrower channel forreceiving small diameter flow tubes 20285. The molded element 20250 canbe attached to the substrate 20220 and cap 20200 using adhesive. Theguide elements 20230 provide a barrier to prevent migration of theadhesive into the flow path or onto the sensing element.

The tube 20285, molded element 20250, and cap 20200 can be made of glasssuch as Pyrex®, fused silica, quartz, sapphire, ceramic, epoxy, one ormore polymers such as PEEK (polyetheretherketone), PTFE(polytetrafluoroethylene), or Teflon®, or metal such as stainless steel.Mixtures of different types of glass or mixtures of different polymerscan also be used to manufacture the tube 20285, molded element 20250,and cap 20200. A stainless steel flow tube 20285 can be attached to thedevice with heat transfer paste or fluid. Oil can be added to the jointbetween the tube 20285 and molded element 20250 and/or substrate 20220to enhance heat transfer.

Another embodiment of isolated flow sensor is shown in FIG. 14. Thelocation channel 20340 formed between guide elements 20330 is wider thanthe sensing element 20370 on the substrate 20320. A molded element 20350extends over the guide elements 20330 and forms the bottom and sides ofa flow channel 20380 that fits within the location channel 20340. A cap20300 forms the top of the flow channel 20380. In one embodiment, themolded element 20350 has a flow channel bottom 20352 with a thin region20355 that contacts the sensing element 20370. The remaining flowchannel bottom 20352 is spaced apart from the substrate 20320, formingair pockets to reduce loss of the thermal signal. Sensors with differentsized flow channels 20380 can be made from the same wafer by usingmolded elements 20350 with different sized flow channels 20380. In someembodiments, the molded element 20350 is disposable and replaceable. Thecap 20300 can also be disposable. Additionally, the cap 20300 can have aprotrusion 20305 extending into the flow channel 20380 to alter thedimensions of the flow channel 20380. As shown in FIG. 15, a cap 20400with a protrusion 20405 extending across the flow channel 20480 reducesthe height and overall dimensions of the flow channel 20480. Theinterface between the exterior of the molded element 20450 and theinterior of the location channel 20440 provides accurate and precisealignment of the flow path over the sensing element, regardless of theinterior size of flow channel 20480.

In a further embodiment, shown in FIG. 16, a flow tube 20585 ispositioned over a thin region 20555 in the flow channel bottom 20552 ofmolded element 20550. The flow tube 20585 forms the flow channel 20580.The molded element 20550 is aligned over the sensing element 20570 bythe guide elements 20530. Cap 20500 contacts the top of the flow tube20585 and maintains the flow tube 20585 in position. In someembodiments, adhesive is also used to maintain flow tube 20585 inposition. In one embodiment, the flow tube 20585 is disposable andreplaceable. This embodiment is particularly suited for analyzing fluidsthat are toxic, corrosive, hazardous, contaminated, etc.

Another embodiment of isolated flow sensor is shown in FIG. 17. Analignment layer is deposited onto a substrate 20620 and a single guideelement 20630 is formed in the alignment layer. The guide element 20630is adjacent the sensing element 20670 on the substrate 20620. A moldedelement 20650 having a first side 20652 a second side 20654 and a flowchannel 20680 therebetween is positioned such that the first side 20652extends over the guide element 20630 to align the flow channel 20680over the sensing element 20670. A cap 20600 forms the top of the flowchannel 20680. In one embodiment, the molded element 20650 has a flowchannel bottom 20652 with a thin region 20655 that contacts the sensingelement 20670. In some embodiments, the remaining flow channel bottom20652 is spaced apart from the substrate 20620, forming air pockets20661 to reduce loss of the thermal signal.

FIG. 18 depicts a graph illustrating the performance of flow sensorswith salt water at ambient temperature, in accordance with an embodimentof the present invention. FIG. 18 indicates that measured flow sensoroutput versus flow for several flow channel configurations and heatertemperature values can be obtained. As illustrated in the graph, flowsthat occur below 0.5μL/min are measurable for a smaller core tube ofonly 150μm internal diameter. In such instances, noise levels may beapproximately in the 1mV range, for which no compensation forfluctuations in ambient temperatures may be in place. Those skilled inthe art can thus appreciate that the graph illustrates a range of datacollected over time regarding nulled-output versus flow rate. FIG. 18thus generally illustrates the beneficial influence of lower wallthickness (WT) and higher thermal conductivity materials for the coretube, which increases sensitivity and flow ranges. An example of ahigher thermal conductivity material, which may be utilized inassociation with an embodiment of the present invention, is Pyrex®.(Note that Pyrex® is a registered trademark of the Corning Glass WorksCorporation of Corning, N.Y. 14831.)

FIG. 19 depicts a graph illustrating the performance of flow sensorswith stainless steel flow tubes on a Pyrex® microbrick with and withoutoil. The stainless steel flow tube had an inner diameter of 0.004inchesand an outer diameter of 0.008inches. Water was used as the fluid. Asshow in the graph, a drop of oil added to the joint between the flowtube and substrate enhanced heat transfer and increased the signalapproximately two-fold.

The flow sensor package disclosed herein offers several advantages overprior art liquid flow sensor packaging approaches. For example, theapplication of reliably controlling the thickness of the insulatinglayer, molded element, or flow tube can eliminate electrical leakagesand the risk of electrical shorts. This controlled thickness alsoenables the application of larger voltages to the sensor heatingelements, thus higher heater temperatures, and thus leads to largeroutput signals, reduced effect of sensor and electronic offsets andwithout boiling the liquid. An isolated flow channel located above thechip cuts down on flow noise while providing the aforementionedbenefits, including eliminating the risk of fluid leakage or corrosionand, additionally, providing electrical insulation of the chip contacts.In addition, the isolated flow channel can provide a “clean”,contaminant-free environment for preserving the maximum fluidcleanliness.

Thus, according to the invention described herein, a sensor can beconfigured to generally include a flow path formed over a sensor chipfor sensing fluid flow, wherein a fluid in the flow path surrounds thesensor chip. Alternatively, the sensor chip can be isolated from theflow path by a flow tube or molded element, which provides electricalinsulation and corrosion protection to the sensor chip, reduces flownoise, essentially eliminates the risk of fluid leakage, and maintainsthe fluid super-clean and contamination-free while improving structuralintegrity for the thermal measurements derived from the sensor chip. Theuse of such an isolated configuration also can protect the sensor fromcorrosion, radioactive or bacterial contamination, deposits,overheating, or freeze-ups. Such an isolated configuration also enablesthe flow tube and/or molded element to be detachable and disposable,without requiring the replacement of the more costly sensor chip and itsassociated electronics.

The flow path is precisely aligned over the sensor chip by an alignmentlayer that forms a location channel. The location channels can be formedon a substrate at the wafer level, providing an inexpensive, efficientmeans of producing multiple sensors with identically aligned flow paths.

The present invention can be used in glucose monitoring, laboratory on achip, drug delivery, cytometer, fluid flow, dialysis, infusion, andother applications. Further, the present invention is applicable tomicrofluidics and flow sensing applications that need to measureliquids, condensing air or contaminated air.

The embodiments and examples set forth herein are presented to bestexplain the present invention and its practical application and tothereby enable those skilled in the art to make and utilize theinvention. Those skilled in the art, however, will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. Other variations and modifications ofthe present invention will be apparent to those of skill in the art, andit is the intent of the appended claims that such variations andmodifications be covered. The description as set forth is not intendedto be exhaustive or to limit the scope of the invention. Manymodifications and variations are possible in light of the above teachingwithout departing from the scope of the following claims. It iscontemplated that the use of the present invention can involvecomponents having different characteristics. It is intended that thescope of the present invention be defined by the claims appended hereto,giving full cognizance to equivalents in all respects.

1. A method of making a plurality of flow sensors each having a flowchannel aligned with a sensing element, the method comprising: providinga substrate having a plurality of sensing elements aligned in a pattern;depositing a polymer alignment layer onto the substrate; forming byphotolithography a plurality of guide elements in the alignment layer,wherein each guide element is aligned adjacent the sensing element;dicing the substrate into a plurality of chips, wherein each chip hasthe sensing element and at least one guide element.
 2. The method ofclaim 1, wherein the step of forming a plurality of guide elementsincludes depositing a polymer layer onto the substrate and forming theguide elements in the polymer layer.
 3. The method of claim 2, furthercomprising applying a cap over the polymer layer on each chip.
 4. Themethod of claim 2, wherein the polymer layer is a photoresist.
 5. Themethod of claim 4, wherein the resist is poly(methyl methacrylate). 6.The method of claim 4, wherein the resist is SU-8.
 7. The method ofclaim 1, wherein the flow channels are formed by etching.
 8. The methodof claim 1, wherein the substrate is selected from the group consistingof quartz, silicon, ceramic, glass, metal, and polymer.
 9. The method ofclaim 1, wherein each guide element defines a flow channel, wherein thestep of forming a plurality of guide elements includes aligning the flowchannels over the sensing elements.
 10. The method of claim 9, furtherincluding the step of providing a plurality of flow tubes incommunication with the flow channels.
 11. A method of making a pluralityof flow sensors each having a flow channel aligned over a sensingelement, the method comprising: (a) providing a substrate having aplurality of sensing elements aligned in a pattern; (b) depositing analignment layer onto the substrate; (c) forming a plurality of guideelements in the alignment layer, wherein at least one guide element isaligned adjacent each sensing element; (d) dicing the substrate to forma plurality of pieces, each piece having a sensing element and at leastone guide element; and (e) placing a molded element containing a flowchannel on the guide element of each piece such that the flow channel isaligned over the sensing element.
 12. The method of claim 11, whereinthe guide elements are substantially the same distance from each sensingelement.
 13. The method of claim 11, wherein step (e) involves placing adifferent size flow channel on each piece to achieve a plurality of flowsensors, each having a different sized flow channel aligned over thesensing element.
 14. The method of claim 11, wherein the alignment layeris a polymer and the guide elements are formed by photolithography. 15.The method of claim 11, further comprising placing a cap on the moldedelement, wherein the molded element forms a bottom and sides of the flowchannel and the cap forms a top of the flow channel.
 16. The method ofclaim 15, wherein the cap includes a protrusion extending into the flowchannel to reduce the volume of the flow channel.
 17. The method ofclaim 15, wherein the cap is attached to the molded element before themolded element is placed on the guide elements.
 18. The method of claim11, wherein the molded element includes a flow channel region and atleast one outwardly protruding arm, wherein the at least one arm extendsover the guide element and the flow channel region is aligned over thesensing element, wherein the molded element is sized such that at leasta portion of a bottom surface of the flow channel contacts thesubstrate.
 19. The method of claim 18, wherein a bottom wall of the flowchannel has a thin area in contact with the substrate over the sensingelement, wherein the remaining area of the bottom wall is spaced fromthe substrate.